Space-charge neutralized electron gun



March 29, 1966 A. EICHENBAUM SPACE-CHARGE NEUTBALIZED ELECTRON GUN FiledFeb. 8, 1963 JMKMM United States Patent 3,243,640 SPACE-CHARGENEUTRALIZED ELECTRON GUN Arie L. Eichenbaum, Levittown, Pa., assignor toRadio Corporation of America, a corporation of Delaware Filed Feb. 8,1963, Ser. No. 257,180 16 Claims. (Cl. S15- 3.5)

This application is a continuation-impart of my application, Serial No.38,780, tiled June 27, 1960, now abandoned, assigned to the sameassignee.

The present invention relates to electron beam tubes, and particularlyto a beam tube having a space charge neutralized electron gun capable ofproducing a stable electron beam of high current density at moderatecathode temperature or a stable electr-on beam of moderate currrentdensity at very low cathode temperature.

The current density :of -a beam consisting essentially of electronscontinuously emitted from .a ilat surface is limited to about 8 amperesper square centimeter (a/ cm.2). Previous attempts to increase the beamcurrent density led to development -of convergent guns of the well knownPierce and Heil types. The beams produced by these guns, while havinghigh current densities, are known to be very noisy and also subject toexcessive beam scalloping. Attempts to avoid these difficulties have ledto the development of hollow cathode guns. While the beam noisiness :andscalloping are reduced with such guns, it was found that space chargelimitati-ons prevented beam current densities in excess of about 8a./cm.2. It is known that much higher electron current densities can beproduced in the electron stream of a triode by introducing positive ionsinto the stream in suicien-t quantity to neutralize the negative spacecharge -of the electrons. Due to lower mobility, and usually higherindividual positive charge, the number of positive ions required forneutralization is much smaller than the number of electrons emitted bythe cathode. APositive ions have been produced by several methods,including (l) ionization of gas atoms as a result of collision or impactby the electron-s in a beam, (2) ionization of allcali metal vapor atomsby contact with a heated metal surface having a work function higherthan the ionization potential of the gas atotms, and (3) directthermionic emission of positive ions from a heated lion source. The rstof these methods is not sui-table for the production of stable highdensity beams or low velocity electron beams (i.e. lower than theionization potential of the gas) useful for low-noise purposes.Therefore, either of the other two methods, which involve positive ionemission, is used in practicing the invention. The second methodinvolving contact ionization is carried out by causing alkali metalatoms, Isuch as cesium, to contact a heated cathode of tungsten, forexample, or by causing the cesium atoms to contact an auxiliary heatedtungsten electrode.

An object of the present invention is to provide an electron tube having.a space charge neutralized electron gun capable of producing a stable,high density beam (at least 20 a./cm.2) .at moderate cathodetemperatures (at least l000 K.). Another object is to provide anelectron tube having a space charge neutralized hollow cathode electrongun capable of producing a stable, relatively dense electron beam (atleast 100 rma/cm?) at low cathode temperatures, of the order of 700 to1000" K., for use in very low noise applications.

These land other objects are accomplished in accordance with theinvention by providing fa thermionic cathode, having a relatively largeemissive surface, land a beam forming electrode, having a single smallbeam aperture with an area that is small compared to the area of theemissive surface of the cathode, located in frront of the 3,243,640Patented Max'. 29, 1966 ice cathode, heating the emissive surface of thecathode to electron emitting temperature, introducing suiiicientpositive ions into the space between the emissive surface and theaperture other than by ionization by electron impact to neutralize thespace charge yof the electrons, and extracting the electrons emittedfrom the emissive surface through the small aperture at low velocity. Aplasma, or substantially neutral mixture of charged particles, is formedrand maintained in this space which makes it possible to bias theIapertured beam forming electrode at a low positive potential relativeto the potential of the emissive surface :and still draw outsubstantially all of the emitted electrons through the beam aperture.Under these conditions, the electron current issuing from the apertureis substantially :a thermal current; that is, its electron temperatureis substantially the same as the cathode temperature. In .other words,the velocity spread yof the electrons in the beam is substantially equalto the velocity spread of the electrons at the emissive surface of thecathode. This thermal current issuing from the aperture at low velocitycan be accelerated to high velocities without substantially increasingthe electron temperature or beam noise, and hence, is hi-ghly suited foriuse in beam ampliers such as klystrons .and traveling wave tubes wherehigh s-ignal-to-noise ratios are desi-red. The positive ions may beproduced by contact ionization of alkali metal vapofr atoms, either bythe electron emissive surface of the cathode or by the surface of ,aseparate metal element spaced from that surface. This is considered anion emission process. Alternatively, the positive ions may be suppliedby la separate thermionic positive ion emitter. Suitable means areprovided for constraining or focusing the beam to prevent spreadingthereof after emerging from the aperture electrode. For high vacuum useof the beam, suitable means, such as a liquid air trap, is provided forreducing the gas pressure in the beam path beyond the :aperturedelectrode to a good vacuum.

In the accompanying drawing:

FIG. l Iis an `axial sectional view of an electron beam tubeincorporating the present invention;

FIG. 2 is a transverse sectional view taken on the line 2 2 of FIG. 1;

FIG. 3 is a graph to be used in explaining the operation of the tube ofFIG. l;

FIGS. 4 and 5 are axial sectional views of two modifications of theelectron gun structure yof FIG. l; and

FIG. 6 is an axial sectional View of another embod-iment of theinvention.

The beam tube shown in FIGS. l and 2 comprises a vacuum tight envelopeor bulb 1 including a cup-shaped end portion 3 containing an electrongun 5 embodying one form of the invention. The gun 5 comprises a hollowtubular thermionic cathode 7 having a relatively large cylindricalinternal electron emissive surface 9 coaxial with the central axis ofthe envelope. The cathode 7 is closely surrounded by a coil 11 forheating the cathode surface 9 to electron emitting temperature. Abeamforming plate electrode 13 having a small central aperture 15 islocated closely adjacent to the upper open end of the cathode 7, withthe aperture 15 coaxial with the cylindrical emissive surface 9. Thearea of the aperture 1S is small compared to the area of the emissivesurface 9.

The gun 5 further includes a positive accelerating electrode 17 having abeam aperture 19 spaced from and coaxial with the aperture 15. Theaperture 19 is somewhat larger than the aperture 15. The electrode 17 ismade of magnetic material and is cup-shaped with a tubular Wall portion20 extending back over the cathode 7 to substantially shield the latterfrom external magnetic fields. The cathode 7, heater 11, plate electrode13 and accelerating 3 electrode 17 are provided with external leads forthe application of suitable operating potentials, a set of which isshown on the drawing as an example.

A small amount of cesium, or a compound containing cesium, is introducedinto the envelope portion 3 during manufacture in any suitable manner.For example, the envelope portion 3 may originally include a tubularappendage containing a number of pellets of cesium generating materialwhich are heated by RF to cause them to explode and inject cesium intothe envelope portion 3, after which the appendage is pinched off asshown at 21. During operation of the tube, the heat from the heater 11may be sufficient in many cases to heat the envelope portion 3 tovaporize the cesium and maintain the desired cesium vapor pressure.However, it is preferred to independently heat the envelope portion 3,as by means of a heater coil 23 surrounding the same, to control thecesium temperature and vapor pressure. The stem portion of the bulb iskept at a temperature somewhat higher than the adjacent cylindricalportion by the heat refiected downward from the hot cathode region. Thiseliminates the necessity for separately heating the stem portion andalso reduces leakage currents between the tube leads by preventing thecondensation of cesium thereon. The excess cesium in the tube isindicated schematically by the numeral 25 in FIG. l.

As shown in FIG. 1, the tube envelope 1 further includes an end portion27 which may contain any desired structure for utilizing the highdensity beam from gun 5, as schematically illustrated by a drift tube 29axially aligned with the beam path. For example, the drift tube 29 maybe the helix or other delay line for traveling7 wave interaction withthe electron beam. Since the interaction region of the tube mustnormally be a high vacuum region, a portion of the beam path between thegun and end portion 27 is surrounded by suitable means, such as a liquidair trap 31, for condensing the cesium vapor thereon to reduce the gaspressure in the interaction region to a good vacuum. The trap 31 issealed to the envelope portions 3 and 27 to form part of the vacuumenvelope of the tube.

The entire tube is surrounded by means such as a magnet coil 33 forestablishing an axial beam focusing magnetic field along the beam pathbeyond the aperture 19. The space within the cup-shaped magnetic member17 is substantially shielded thereby from the field of coil 33. Whilethe magnetic shield 17 causes most of magnetic ux from coil 33 toby-pass the space within the hollow cathode 7, some of the flux willthread through this space and thereby assist in directing the electronsthrough the exit aperture 15.

In the operation of the tube, the cathode 7 is heated to the desiredtemperature, by the heater 11 and an external current source 35, tocause the surface 9 to emit a copious flow of electrons. These electronsare extracted through the small apertures and 19 by a low positiveaccelerating potential on the plate electrode 13 and/or the field of arelatively high positive potential on electrode 17. In order to overcomespace charge effects and concentrate and pass most of the electronsemitted by the large area hollow cathode through the small aperture 15,sucient positive ions are provided within the hollow cathode toneutralize the space charge of the electrons. This is done by heatingthe gun enclosure, as by means of the heater coil 23, to a temperaturesuicient to vaporize some of the cesium and maintain the desired cesiumtemperature and vapor pressure. The cesium vapor atoms diffusethroughout the gun enclosure including the interior of the hollowcathode. The surface 9 of the cathode 7 is made of a high work functionmetal such as tungsten. Some of the cesium atoms which come into contactwith the hot tungsten surface 9 give up electrons to that surface by thephenomena of contact ionization and become free positive ions. This isbecause the ionization potential of cesium, which is 3.9 volts, is lessthan the electron work function, 4.5 volts, of pure tungsten. It isbelieved that the contacting vapor atoms share electrons with thesurface, and hence, they are effectively part of the surface. The heatof the surface causes the ions to be emitted therefrom leaving theshared electrons in the surface. 1f the cathode temperature is highenough relative to the cesium temperature, none of the cesium atoms orions remain on the tungsten surface. In this case, the effective workfunction of the emissive surface 9 is that of pure tungsten, 4.5 volts.At lower cathode temperatures for a given cesium temperature, some ofthe cesium atoms are adsorbed by the tungsten surface, which reduces theeffective work function of some areas of the tungsten surface whileleaving other areas of the tungsten surface bare for contact ionizationof cesium atoms.

Data on the emission of electrons from cesiated tungsten cathodes werepublished by Langmuir and Kingdon, Physical Review 21, p. 380, 1923; andTaylor and Langmuir, Physical Review 44, p. 423, 1933. In theseexperiments, the emission due to adsorbed layers of cesium on tungstenWire was measured as a function of the vapor pressure of saturatedcesium vapor (given by the bulb temperature) and the tungsten cathodetemperature. FIG. 3 shows typical curves obtained by Taylor and Langmuirfor cathode temperatures between about 500 and 2200 K., and bulbtemperatures of 253, 270, 290, 313, 340, 372, and 412 K. The graph alsoincludes lines indicating the fractional coverage, 0, of tungsten bycesium, for values of 0, .2, .4, .5, .55 and .67. It can be seen that,for each bulb temperature, as the cathode temperature is lowered from arelatively high value, the emission first decreases to a minimum at alow value of 6, then increases to a maximum at about 9:.55, and thendecreases again. All of the curves merge at the 9:0 line, so that highelectron emission can be obtained at high cathode temperature for eachvalue of bulb temperature. It is believed that at the 0:.55 maximum thetungsten is covered by substantially a monomolecular layer of cesiumatoms. At this value of 0 the effective work function of the cesiatedtungsten surface is about 1.8 volts.

The tube of FIG. l can be operated with any combination of cathode andbulb temperatures deterimning values of 0 between 0:0 and 9:1. Thehighest beam densities are obtained with cathode temperatures above 2200K. and at 9:0. In such higher temperature operation, electrons areemitted copiously by the cathode and the entire tungsten surface isavailable for contact ionization of the cesium (0:0). For a particularemission rate, in this case determined solely by the cathodetemperature, the supply of positive ions is regulated by adjusting thebulb temperature to obtain maximum beam current through the aperture 15,which means best obtainable neutralization of the space charge. Underthese conditions the mixture of electrons and positive ions within thecathode 7 constitutes a plasma that extends substantially to the exitaperture 15. If the effective potential of the beam forming electrode 13is the same as the cathode 7, and the accelerating electrode has asubstantial positive bias, the plasma will extend to a region justbeyond the aperture 15, and a high density, thermal electron currentwill be drawn from the boundary of the plasma by the applied fields. Ifthe effective potential of electrode 13 is substantially higher than thecathode potential, the plasma boundary will be depressed below theaperture 15, but the current extracted through the aperture will stillbe substantially thermal. On the other hand, conventional electron gunswithout space charge neutralization produce either low temperature beamswith very low densities or high density beams with very high electrontemperatures. In one tube designed for high temperature operation andhaving a tungsten cathode similar to that of FIG. 1, except that it wasdirectly heated, with a length of about mills and an inner diameter ofabout 500 mils, and a plate aperture 15 of about 30 mils diameter, thushaving a ratio of emissive area to aperture area of about 400 to l, Ihave obtained a beam having a current density at the aperture of about40 amperes/cm-2, at a cathode temperature of about 2200" K. This isconsiderably lower than the usual operating temperature of tungstenlaments, which is about 2500 K. A similar tube having an area ratio of1000 to 1 operated with the cathode at 2200 K. should have a beamcurrent density of about 100 amperes/cm-2. A similar tube having a ratioof emissive area to aperture area of about 140 to 1 operated at acathode temperature of about 2400 K. has produced a beam current densityof about 70 amperes/ cm?, In both of these high temperature examples,the plate electrode 13 was operated essentially at cathode potential.Preferably, the active cathode area should be at least two orders ofmagnitude (of the order of 100 to 1) greater than the aperture area.

The particular metals, tungsten and cesium, described above, are merelyexamples. Instead of tungsten, the cathode may be made of tantalum (workfunction 4.07 volts), molybdenum (4.3 volts) or oxidized tungsten (9.2volts), all of which have relatively high work functions are suitablefor use in the tube at the desired temperatures. Other alkali metalssuitable for use instead of cesium are rubidium (ionization potential4.17 volts) and potassium (4.34 volts). Of course, the alkali metal usedmust have an ionization potential lower than the particular cathodematerial selected. Although the curves in FIG. 3 apply only to tungstenand cesium, similar curves may be drawn for other combinations, such asnickel and 'cesium, for example, since the reactions between the alkalimetals and the cathode materials are similar, with respect to bothcontact ionization and reduction of cathode work function.

For many purposes, such as in low noise traveling Wave tubes, it isdesirable to operate at very low cathode temperatures, with lowercurrent densities. However, at low cathode temperatures the ionizationeciency of the cathode is low, not only because of the low temperatureitself but also because the cesium coverage interferes with theproduction of ions by contact ionization at the cathode surface, whichtends to result in incomplete neutralization of the electron spacecharge. Therefore, in low temperature operation, electrode 13 should beoperated at a small positive potential (e.g., one Volt, as shown in FIG.l) to provide a small accelerating eld near the exit aperture 15, whicheld assists in extracting the beam. Even with this small acceleratingeld the electrons emerging from the aperture 15 constitute a thermalcurrent since the velocity spread in the electron stream is equal tothat at the cathode.

I have operated an electron gun constructed as shown in FIG. l, with atungsten cathode cylinder having a length and diameter of about 500mils, and a plate 13 having an aperture 15 of about 30 mils diameter,thus having a ratio of active cathode area to aperture area greater than1000 to l, at a cathode temperature of about 760 K. (only 487 C.) and acesium pressure determined by a bulb temperature of about 313 K.,obtaining a beam current density of about 100 ma./cm.2, which is amplefor most low noise applications. As in the case of the highertemperature operation described above, tantalum, molybdenum, andoxidized tungsten can be used instead of tungsten for the cathode. Also,lower melting point emissive materials such as nickel (work function 5volts) or a sintered mixture of powders of tungsten and barium oxide(2.7 volts, average) can be used for low temperature operation inFIG. 1. Also, rubidium or potassium can be used instead of cesium withany of these alternative cathode materials. As an example, in a tubehaving a nickel cathode 7 operated at about 825 K. in the presence ofcesium, a beam current density of about 2a/cm.2 was obtained.

Since the positive ions are produced at the cathode surface 9 in FIG. l,the ion density within the cathode will not be uniform throughout theelectron paths. Thus, it

6 may not be possible to obtain complete neutralization of the spacecharge under all conditions of operation.

FIG. 4 shows a modification of the electron gun of FIG. 1 wherein thepositive ions are produced at the surface of a separate ionizingelement, shown as a coil 37 of tungsten, for example, coaxially mountedWithin the hollow cathode 7, instead of at the surface 9 of the cathode.Any of the alkali metals cesium, rubidium and potassium may be providedwithin the envelope as in FIG. l to be contact ionized by the coil 37.For example, for use in a 1/2 x 1/2 cylindrical cathode, the coil 37 maybe made of 10 mil diameter wire with 40 mils spacing between turns andhave a diameter of mils. Preferably, coil 37 is biased about 2 voltspositive with respect to the cathode 7, to accelerate the electronstoward the coil 37 and accelerate the positive ions toward the cathode7,

and a positive potential of about one volt is applied to the plate 13 toestablish a small axial eld component toward the exit aperture 15.

The structure of FIG. 4 is particularly suitable for low noiseoperation, in which case the cathode is maintained, by coil 35, at avery low temperature and the coil 37 is maintained, by a current source39, at a higher temperature. The supply of positive ions forneutralization of the space charge is controlled by both the temperatureof the coil 37 and the vapor pressure of the cesium vapor (by coil 23).Since the coil 37 is for providing ions only, it is preferably operated-under conditions for maximum ion production, i.e. with 0:0. Forexample, a gun having a tungsten cathode 7 and an aperture 15 having anarea ratio of about 1000 to l, a tungsten coil 37 and cesium vapor at acathode temperature of about 760 K., a coil temperature of about 1600"K. and a bulb temperature of about 313 K., produced a low temperaturelow velocity electron beam having a current density at the aperture 15of about 150 ma./crn.2, which was 50% better than current densityproduced by the gun of FIG. l under similar conditions without the coil37. In this example, the electron emission from the cesiated tungstencathode is a maximum in the low temperature region, for the particularcesium pressure used (see FIG. 3).

Since the structure of FIG. 4 does not depend upon contact ionization atthe cathode surface 9 for the production of positive ions, any suitablecathode material can be used instead of cesiated tungsten or the othercathode niaterials listed above, such as thoriated tungsten (workfunction 2.6 volts) or oxide coated cathodes (about 1.1 volts). It willbe understood that the structure can also be operated under conditionsof higher cathode temperature, to produce very high density beams, asdescribed for FIG. 1.

FIG, 5 shows a further modification of the gun of FIG. 1, in which thepositive ions are thermionically emitted directly by a positive ionsource 41 into the path of the electrons to be space charge neutralized.The source 41 may comprise a heater coil 43 embedded in or completelycovered by a fused mass 45 of a material known as eucryptite,Li2OAl2O32SiO-2, having the following composition:

a-type, from which no ion emission takes place, and a high temperature-type which emits positive lithium ions. The a-type is converted intothe -type at 972 C.i20 C.

As an example, the coil 43 may be a coil of 8 mil diam. Wire ofrhodium-platinum alloy with 40 turns per inch, coated with eucryptitematerial 45 to a diameter of 150 mils, for use in a 1/2" x 1/z cathodecylinder 7. The temperature of the eucryptite material 45 is adjusted,in the range from 1100 to l500 K., by varying the current in the coil 43by an external current source 47, to produce 7 the maximum beam currentdensity beyond the apertures and 19.

In operation, the ion emitter coil 43 should be biased at a positivepotential somewhat higher than the potential of coil 37 in FIG. 4,because the mass 45 is insulating, in order that the surface of mass 45will have the desired potential of about 2 volts. The plate 13 ispreferably biased at about one volt positive, as in FIG. 4.

The emissive surface 9 of the hollow cathode 7 in FIG. 5 may be made ofany suitable electron emissive material, as in the embodiment shown inFIG. 4. For example, a gun having a barium oxide coated cathode 7 at atemperature of 1100" K., with a ratio of active cathode area to aperturearea of about 1000 to 1, should produce a high density beam having acurrent density of at least 100 a./cm.2. For low temperature use, asimilar gun having a cesiated tungsten cathode 7 at a cathodetemperature of 760 K. with a bulb temperature of 313 K., where 6:.55,should give a low temperature beam with a current density of about 300ma./cm.2. It will be understood, that the bulb heater coil (23 ofFIG. 1) may be omitted in the modification of FIG. 5, except in the casewhere an alkali metal vapor is used to lower the work function of thecathode material,

FIG. 6 shows another modification of the electron gun in FIG. 1 in whichthe positive ions are produced at th-e surface of an ionizing elementseparate from the cathode. In this embodiment of the invention, theelectron gun comprises a thermionic cathode 51, a combinationbeamforming and ion-producing electrode 53 having a heater coil 55 and acentral aperture 57, an apertured heat shield 59 having a skirt portion61 surrounding the electrode 53, and a cup-shaped apertured acceleratingelectrode 63 within which the other electrodes are mounted.

The cathode 51 comprises a circular electron emissive surface 52, facingelectrode 53, having an effective work function from 1 to 3 volts. Thelower values (1-2 volts) apply to cathodes such as oxide-coated cathodesand cesiated-tungsten cathodes operating in the temperature range 800 to1200 K., while the higher values (2-3 volts) apply to impregnatedcathodes and L-cathodes operating in the temperature range 1100 to 1500K. The surface 52 may be flat as shown, or slightly concave for focusingpurposes.

The surface 54 of electrode 53 facing the cathode 51 is designed tosupply sufficient positive ions to the space between the cathode 51 andthe aperture 57 to neutralize the space charge of the electrons emittedby the surface 52. For example, the ions may be introduced by contactionization of alkali metal vapor atoms, such as cesium, by the heatedsurface 54, as in FIGS. 1 and 4. In such case, the surface 54 is made ofa material such as tungsten, tantalum, hafnium, molybdenum, niobium orcarbon having a work function higher than the ionization potential ofthe alkali metal vapor used. Electrode 53 is spaced from the cathodesurface 52 a sufficient distance to form a substantial plasma regiontherebetween. For example, the emissive surface 52 may have a diameterof 250 mils, the aperture 57 may have a diameter of 30 mils, and thespacing between the surface 52 and aperture 57 may be 80 mils, in whichcase the ratio of active cathode area to aperture area is about 70 to 1.

The cup-shaped accelerating electrode 62 is made of magnetic material inorder to substantially shield the interior space from external magneticfields as in FIG. 1.

In operation, the cathode is heated by a conventional heater (not shown)to a temperature producing copious emission of electrons, and thesurface 54 is heated by heater 55 to a temperature in the range1100-1500 K. to produce efficient production of ions by Contactionization. The vapor pressure of the alkali metal vapor, preferablycesium, is controlled, eg., by the envelope temperature as in FIG. l, tomaintain an adequate supply of atoms for ionization. When the cathode 51and electrode 53 are operated at nearly the same potential, theelectrons and ions mix to form a plasma in the region between theemissive surface 52 and the aperture 57 from which a high densitythermal electron current can be extracted through the aperture. Withzero bias applied to the electrode 53, the latter is slightly negativewith respect to the cathode, due to the fact that the effective workfunction of surface 54 is slightly higher than that of the cathode.Thus, the surface 54 repels the electrons in the plasma. The potentialat the exit aperture 57 is slightly higher than surface 54 due toaccelerating fields provided by electrodes 59 and 63. On the other hand,the positive ions produced by the negative surface 54 are attracted bythe negative space charge which results in efficient mixing of the ionswith the electrons to form the plasma. Preferably, the effectivepotential of the surface 54 is only slightly negative (a few tenths of avolt) relative to emissive surface 52, in which case the plasma regionis generally cone-shaped with its apex in the aperture 57, the potentialat the aperture 57 is zero or only slightly positive, and the aperturecurrent is substantially thermal. The bias potentials V1, V2 and V3, onelectrodes 53, 59 and 63, are chosen to produce this condition. In atube having the electron gun of FIG. 6, with substantially thedimensions given in the above example, and with a barium-impregnatedtungsten cathode operated at about 1400" K. and having a work functionof about 2.2 volts, and an ion-producing surface 54 of niobium operatedat about 1350 K. with electrodes 53, 59 and 63 operated at about 0, +4,and +4 volts, respectively, a beam current density at aperture 57 of 28a./cm.2 was obtained. By measuring the current drawn by each of theelectrodes, it was determined that about of the total electron currentfrom surfaces 52 and 54 was drawn through the aperture 57. In thisexample, the niobium surface 54 included portions covered by cesiumhaving a work function of about 2 volts. Thus, these portions emitelectrons which also enter the plasma region and contribute to theextracted beam current. The positive ions are produced by contactionization at the bare portions of the surface 54, which portions havean ionizing work function of about 4 volts. The effective work functionof the surface seen by the electrons and ions at an appreciable distancetherefrom may be about 3 volts, for example, in which case the effectivepotential of the surface 54 relative to the cathode surface 52 would be(-3)-(-2.2), or .8 volt, if V1 were zero.

In each of FIGS. 1, 4, 5 and 6 the bias potential on the beam formingelectrode is preferably not greater than about 1 volt, and in any casenot greater than the ionization potential of the alkali metal vaporused, which is about 4 volts for cesium.

The aperture in the beam forming electrode in each embodiment need notbe completely unobstructed, as shown in the drawings. For example, afine mesh grid may be mounted across the aperture, to maintain the samepotential across the aperture plane.

What is claimed is:

1. An electron gun comprising:

(a) a plurality of electrodes including (l) a thermionic cathode havinga relatively large electron emissive surface, and

(2) a beam forming plate electrode adjacent to said cathode having asingle aperture spaced from said surface and having an aperture areathat is small compared to the area of said surface;

(b) means, including one of said electrodes, for introducing positiveions into the space between said surface and said aperture other than byionization by electron impact, for neutralizing the space charge ofelectrons emitted by said surface and thereby forming a plasma in saidspace; and

(c) means, including an apertured positive accelerating electrodecoaxially positioned adjacent to said apertured beam forming electrode,for extracting a dense,

substantially thermal electron current from said plasma through saidaperture.

2. An electron gun comprising:

(a) a plurality of electrodes including l) a thermionic cathode having arelatively large electron emissive surface, and

(2) a beam forming plate electrode adjacent to said cathode having asingle aperture spaced from said surface and having an aperture areathat is small compared to the area of said surface;

(b) means for introducing positive ions into the space between saidsurface and said aperture for neutralizing the space charge of electronsemitted by said surface and thereby forming a plasma in said space, saidmeans comprising (l) an ion emitter having a relatively high workfunction, and

(2) means for maintaining alkali metal vapor atoms having an ionizationpotential lower than said work function at a surface of said ion emitterfor contact ionization thereby; and

(c) means, including an apertured positive accelerating electrodecoaxially positioned adjacent `to said apertured beam forming electrode,for extracting a dense substantially thermal electron current from saidplasma through said aperture.

3. An electron gun comprising:

(a) a thermionic cathode having a relatively large active surface, oneportion of said surface having a low effective Work function foremitting electrons at relatively low temperature-s, another portion ofsaid surface having a high work function;

(b) a beam forming electrode adjacent to said cathode havin-g a singleaperture spaced from said surface and having an aperture area that issmall compared to the area of said surface;

(c) means for maintaining alkali metal vapor atoms having an ionizationpotential lower than said high work function at said other portion ofsaid surface for producing positive ions by contact ionization thereby,to neutralize the space charge of said electrons and thereby form aplasma in the space between said surface and said aperture; and

(d) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a dense substantially thermal electron current from saidplasma through said aperture.

4. An electron gun comprising:

(a) a plurality of electrodes including l) a thermionic cathode having arelatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a singleaperture spaced from said surface and having an aperture area that issmall compared to the area of said surface;

(b) means for introducing positive ions into the space lbetween saidsurface and said aperture for neutralizing the space charge of electronsemitted by said surface and thereby forming a plasma in said space, saidmeans comprising 1) an ion emitter adjacent to fbut spaced froml saidcathode, and (2) means for heating said ion emitter to ionemittingtemperature, and (c) means, including an apertured positive acceleratingelectrode coaxially positioned close to said apertured beam formingelectrode, for extracting a dense substantially thermal electron currentfrom said plasma through said aperture. S. An electron gun comprising:(a) a plurality of electrodes including (1) a thermionic cathode havinga relatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a singleaperture spaced from said surface and having an aperture area that issmall compared to the area of said surface;

(b) means for introducing positive ions into the space between saidsurface and said aperture for neutralizing lthe space charge ofelectrons emitted by said surface and thereby forming a plasma in saidspace, said means comprising (l) an ion emitter adjacent to but spacedfrom said cathode and having a relatively high work function,

(2) means for maintaining alkali metal atoms havin-g an ionizationpotential lower than said work function at a surface of said ion emitterfor contact ionization thereby, and

(3) means for heating said ion emitter to contact ionizing temperature;and

(c) means, including an apertured positive acceleratingv electrodecoaxially positioned close to said apertured -beam forming electrode,for extracting a dense, substantially thermal electron current from)said plasma through said aperture.

6. An electron gun comprising:

(a) a thermionic cathode having a relatively large electron emissivesurface;

(b) a ybeam forming electrode adjacent to said cathode having 1) asingle aperture spaced from said emissive surface and having an aperturearea that is small compared to the area of said surface, and

(2) a surface of high work function surrounding said .aperture andfacing said emissive surface;

(c) means for introducing positive ions into the space between `saidemissive surface and said aperture for neutralizing the space charge ofelectrons emitted Iby said surface thereby forming a plasma in saidspace, said means comprising:

(1) means for maintaining alkali metal vapor atoms having an ionizationpotential lower than said work function at said surface of saidbeamforming electrode for contact ionization thereby, and

(2) means for heating said surface to contact ionizing temperature; and

(d) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a dense, substantially thermal electron current from saidplasma through said aperture.

7. an electron gun comprising:

(a) a plurality of electrodes including (l) a thermionic lcathode havinga relatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a singleaperture spaced from said surface and having an aperture area that -issmall compared to the area of said surface;

(b) means for introducing positive ions into the space between saidsurface and said aperture for neutralizing the space charge of electronsemitted by said surface and thereby forming a plasma in said space, saidmeans comprising (1) an electrode adjacent to but spaced from saidcathode and comprising a body of a material which emits positive ionswhen heated, and

(2) means for heating said -body to ion-emitting temperature; and

(c) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a dense, substantially thermal electron current from saidplasma through said aperture.

8. An electron beam tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least atone end and having a relatively large internal electron emissive surfaceof high work function;

(b) a beam forming plate electrode disposed adjacent to said open endand having a single aperture coaxial with said cylinder, the area ofsaid aperture being at least two -orders of magnitude smaller than thearea of said surface;

(c) means for maintaining alkali metal vapor atoms having an ionizationpotential lower than said work function at said surface for producingpositive ions by contact ionization thereby, to neutralize the spacecharge of electrons emitted by said surface and thereby form a plasma inthe space between said surface and said aperture; and

(d) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a dense, substantially thermal electron current from saidplasma through said aperture.

9. An electron beam tube comprising an envelope containing:

(a) a plurality of electrodes including,

(1) a thermionic cathode comprising a hollow cylinder open at one endand having a relatively large internal electron emissive surface, and

(2) a beam forming plate electrode disposed adjacent to said open endand having a single aperture coaxial with said cylinder, the area ofsaid aperture being at least three orders of magnitude smaller than thearea of said surface;

(b) means, including an ion emitter, for introducing positive ions intothe space between said surface and said aperture, for neutralizing thespace charge of electrons emitted by said surface and thereby forming aplasma in said space; and

(c) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a relatively dense thermal electron current from said plasmathrough said aperture.

10. An electron beamI tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least atone end and having a relatively large internal electron emissivesurface;

(b) a beam forming plate electrode disposed adjacent to said open endand having a single aperture coaxial with said cylinder, the area ofsaid aperture being at least three orders of magnitude smaller than thearea of said surface;

(c) means for introducing positive ions into the space between saidemissive surface and said aperture for neutralizing the space charge ofelectrons emitted by said surface, said means comprising:

(1) an ion-emitter mounted coaxially within said cylinder and spacedfrom said aperture, and

(2) means for heating said ion emitter; and

(d) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a relatively dense thermal electron current from said plasmathrough said aperture.

11. An electron beam tube as in claim 10, wherein said ion emitter is acoil of high work function metal and said ion introducing means furthercomprises means for supplying alkali metal vapor atoms having anionization potential lower than said Work function at the surface ofsaid coil for contact ionization thereby.

12. An electron beam tube asin claim 10, wherein said ion emittercomprises a body of -eucryptite material l2 which emits positive lithiumions when heated above about 1l00 K.

13. An electron gun comprising:

(a) a plurality of electrodes including (1) a thermionic cathode havinga relatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a singleaperture spaced from said surface and having an aperture area that issmall compared to the area of said surface,

(b) means, including an ion emitter, for introducing positive ions intothe space between said surface and said aperture for neutralizing thespace charge of electrons emitted by said surface; and

(c) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a relatively dense substantially thermal electron currentfrom said plasma through said aperture, said accelerating electrodebeing made of magnetic material and comprising a tubular shield portionsurrounding said cathode and said beam forming electrode.

14. An electron beam tube comprising:

(a) an electron gun for producing an electron beam along a predeterminedpath, including (1) a plurality of electrodes disposed along said pathand comprising (A) a thermionic cathode having a relatively largeelectron emissive surface, and

(B) a beam forming electrode adjacent to said cathode having a singleaperture spaced from said surface and having an aperture area that issmall compared to said emissive area,

(2) means, including an ion emitter, for introducing positive ions intothe space between said surface and said aperture, for neutralizing thespace charge of electrons emitted by said surface and thereby forming aplasma said space, and

(3) means, including an apertured positive accelerating electrodecoaxially positioned close to said apertured beam forming electrode, forextracting a dense, substantially thermal electron current from saidplasma through said aperture;

(b) means coupled to said path in a region beyond said electron `gun forinteraction with said beam; and

(c) condensing means, interposed between said electron gun and saidinteraction means, for removing gas from said beam path and therebymaintaining a high vacuum in the interaction region of the tube.

15. An electron beam tube as in claim 14, wherein said condensing meanscomprises a liquid air trap surrounding said beam path.

16. An electron beam tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least atone end and having a relatively large active internal surface, oneportion of said surface having a low eifective work function foremitting electrons at relatively ylow temperatures, another portion ofsaid surface having a high work function;

(b) a beam forming plate electrode disposed adjacent to said open endand having a single aperture coaxial with said cylinder, the area ofsaid aperture being at least two orders of magnitude smaller than thearea of said surface; v

(c) means for maintaning alkali metal atoms having an ionizationpotential lower than said high work 3,243,640 13 14 function at saidother portion of said surface for References Cited by the Examinerproducing positive ions -by contact ionization there- UNITED STATESPATENTS by, to neutralize the space charge of said electrons e d th b f1e eh e between said 2,798,181 7/1957 Foster 313-1611 Sgrfaeeern sffprtme spac 5 2,841,726 7/1958 Kmechui 313-230 X (d) means, including anapertured positive accelerating 218831560 4/1959 Beam et@ 313"320 Xelectrode coaxially positioned close to said aper- 3,021,472 2/1962Hermqulst 313-2305( tured Ebeam forming electrode, for extracting adense, substantially thermal electron current from said plas- HERMANKARL SAALBACH Primary Exammer ma through said aperture. lo S. CHATMON,JR., Assistant Examiner.

1. AN ELECTRON GUN COMPRISING: (A) A PLURALITY OF ELECTRODES INCLUDING(1) A THERMIONIC CATHODE HAVING A RELATIVELY LARGE ELECTRON EMISSIVESURFACE, AND (2) A BEAM FORMING PLATE ELECTRODE ADJACENT TO SAID CATHODEHAVING A SINGLE APERTURE SPACED FROM SAID SURFACE AND HAVING AN APERTUREAREA THAT IS SMALL COMPARED TO THE AREA OF SAID SURFACE; (B) MEANS,INCLUDING ONE OF SAID ELECTRODES, FOR INTRODUCING POSITIVE IONS INTO THESPACE BETWEEN SAID SURFACE AND SAID APERTURE OTHER THAN BY IONIZATION BYELECTRON IMPACT EMITTED BY SAID SURFACE AND THEREBY FORMING A PLASMA INSAID SPACE; AND (C) MEANS, INCLUDING AN APERTURED POSITIVE ACCELERATINGELECTRODE COAXIALLY POSITIONED ADJACENT TO SAID APERTURED BEAM FORMINGELECTRODE, FOR EXTRACTING A DENSE, SUBSTANTIALLY THERMAL ELECTRONCURRENT FROM SAID PLASMA THROUGH SAID APERTURE.